[vorbis-dev] Vorbis bitstream specification...

[Segher Boessenkool]

> Codebooks are general and may need to be anywhere from 1.31 to 24.8.

Don't forget about -100.132 and things like that.

> 3) the MDCT on mainline has a stable dynamic range and is easy to
> integerize.  The mainline integer version assumes a 14 bit fraction on
> the trig lookups, which frankly is far too little.  This is done only
> because C doesn't have a 32x32->64 bit two result register multiply.

#ifndef BIGENDIAN
union magic {
        struct {
                int32 lo;
                int32 hi;
        } halves;
        int64 whole;
};
#else
...
#endif

tatic inline int32 himul(int32 x, int32 y) {
        union magic magic;
        magic.whole = (int64)x * y;
        return magic.halves.hi;
}

and with optimization turned on, everything will work Just Fine(tm).

Also, signed a.b * signed c.d ==> signed (a+c-1).(b+d) (effective range).
You can do tricks with unsigned muls to prevent that -1.

> If you're on ARM (for example) and have SMULL, use 2.30 for the
> lookups, only save the RdHi register and you'll have very good S/N.

Roundoff errors?  You can do it like this, but be careful about signs.
(Truncating in two's complement always rounds towards -inf; make sure
these errors don't accumulate).

> There's nothing tricky in an integerization;

...but there are a lot of tricks in integerization :)

<p>Segher

<p>--- >8 ----
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